The present invention relates generally to the regenerative incineration of solvents and other hydrocarbons in exhaust streams, and more particularly, to a regenerative bed, switching flow-type incinerator for processing waste gas/exhaust air with low hydrocarbon loadings.
Many manufacturing operations produce waste gases or exhaust air which include environmentally objectionable contaminants, generally combustible fumes such as solvents and other hydrocarbon substances, e.g., gasoline vapors, paint fumes, chlorinated hydrocarbons. The most common method of eliminating such combustible fumes prior to emitting the exhaust gases to the atmosphere is to incinerate the waste gas or exhaust air stream.
One method of incinerating the contaminants is to pass the waste gas or exhaust air stream through a fume incinerator prior to venting the waste gas or exhaust air stream into the atmosphere. An example of a suitable fume incinerator for incinerating combustible fumes in an oxygen bearing process exhaust stream is disclosed in U.S. Pat. No. 4,444,735. In such a fume incinerator, the process gas stream is passed through a flame front established by burning a fossil fuel, typically natural gas or fuel oil, in a burner assembly disposed within the incinerator. In order to ensure complete incineration of the combustible contaminants, all of the process exhaust stream must pass through the flame front and adequate residence time must be provided. Additionally, it is desirable to preheat the process exhaust stream prior to passing it through the flame front so as to increase the combustion efficiency. Of course, the cost of the heat exchanger to effectuate such preheating, in addition to the cost of the auxiliary fuel, render such fume incinerators relatively expensive.
Another type of incinerator commonly used for incinerating contaminants in process exhaust streams is the multiple-bed, fossil fuel-fired regenerative incinerator, such as, for example, the multiple-bed regenerative incinerators disclosed in U.S. Pat. Nos. 3,870,474 and 4,741,690. In the typical multiple-bed systems of this type, two or more regenerative beds of heat-accumulating and heat-transferring material are disposed about a central combustion chamber equipped with a fossil fuel-fired burner. The process exhaust stream to be incinerated is passed through a first bed, thence into the central combustion chamber for incineration in the flame produced by firing auxiliary fuel therein, and thence discharged through a second bed. As the incinerated process exhaust stream passes through the second bed, it loses heat to the material making up the bed. After a predetermined interval, the direction of gas flow through the system is reversed such that the incoming process exhaust stream enters the system through the second bed, wherein the incoming process exhaust stream is preheated prior to entering the central combustion chamber, and discharges through the first bed. By periodically reversing the direction of gas flow, the incoming process exhaust stream is preheated by absorbing heat recovered from the previously incinerated process exhaust stream, thereby reducing fuel consumption.
A somewhat more economical method of incinerating combustible contaminants, such as solvents and other hydrocarbon based substances, employing a single regenerative bed is disclosed in U.S. Pat. No. 4,741,690. In the process presented therein, the contaminated process exhaust stream is passed through a single heated bed of heat absorbent material having heat-accumulating and heat-exchanging properties, such as sand or stone, to raise the temperature of the contaminated process exhaust stream to the temperature at which combustion of the contaminants occurs, typically to a peak preheat temperature of about 900.degree. C., so as to initiate oxidization of the contaminants to produce carbon-dioxide and water. Periodically, the direction of flow of the process exhaust stream through the bed is reversed. As the contaminants combust within the center of the bed, the temperature of the process exhaust stream rises. As the heated exhaust stream leaves the bed, it loses heat to the heat-accumulating material making up the bed and is cooled to a temperature about 20.degree. C. to 25.degree. C. above the temperature at which it entered the other side of bed. By reversing the direction of the flow through the bed, the incoming contaminated process exhaust stream is preheated as it passes that portion of the bed which has just previously in time been traversed by the post-combustion, hot process exhaust stream, thereby raising the temperature of the incoming process exhaust stream to the point of combustion by the time the incoming process exhaust stream reaches the central portion of the bed.
In the regenerative bed heat exchanger apparatus disclosed in U.S. Pat. No. 4,741,690, a heating means, typically an electric resistance heating coil disposed in the central portion of the bed, is provided to initially preheat the central portion of the bed to a desired temperature at which combustion of the contaminants in the process exhaust stream would be self-sustaining. When incinerating process gas streams with moderate or high hydrocarbon loadings, once steady state equilibrium conditions are reached, the electric resistance heating coil may usually be deactivated as the incoming process exhaust stream is adequately preheated and combustion is self-sustaining due to the gas switching procedure hereinbefore described.
However, when a process gas stream has a low concentration of combustible contaminants, i.e., a low hydrocarbon loading, there may be insufficient heat content liberated during incineration of the process gas stream to properly preheat the incoming process gas stream. That is, the BTU content of the incinerated process gas stream may be insufficient to heat the material in the downstream portion of the bed to the temperature at which combustion of the contaminants will occur. Accordingly, when treating such low hydrocarbon content process gas streams, it may be necessary to continuously supply current to the electric resistance heating coil disposed in the central portion of the bed so as to ensure that the bed material therein is maintained high enough to ensure that combustion of the insufficiently preheated process gases will be sustained. Unfortunately, due to the cost of electricity, it is uneconomical to incinerate low content process gas streams in such an electrically assisted regenerative bed incinerator.